1. Field of the Invention
The present invention relates to a method of correcting coordinates measured in a coordinate measuring machine. More particularly, it relates to a coordinate measuring system and method of correcting measured coordinates when a work with a weight is mounted on a base in a coordinate measuring machine.
2. Description of the Related Art
A variety of probes are employed in a coordinate measuring machine for measurements within a scale coordinate system, which includes reference scales arranged along axes that configure a three-dimensional measuring space. To provide the coordinate measuring machine with an improved measurement precision, a structure thereof is required to have a higher static stiffness. In addition, introduction of a software spatial precision correcting technology can reduce geometrical errors as low as possible to support higher precision.
The coordinate measuring machine has geometrical errors, including scale errors, straightness errors, and angular errors such as pitching and yawing on axes within an orthogonal coordinate system in a kinematic model, as shown in FIG. 4. These errors are grouped as follows with a total of 21 error factors.
Scale errors on axes:3Horizontal straightness errors on axes:3Vertical straightness errors on axes:3Pitching errors on axes:3Yawing errors on axes:3Rolling errors on axes:3Angular errors between axes:3
In general, as geometrical errors measured on CMM (Coordinate Measuring Machine) contain influential factors such as angular errors, handling these as errors on axial references requires a process of error separation. A technology has been known to execute error separation using a kinematic model as shown in FIG. 4 (see JP-A 7-146130, Paragraphs 0002–0007 and FIG. 4). This kinematic model is not only employed for error separation when a correction parameter is computed. It is also employed for a process of conversion of each correction parameter into an error in a coordinate space when correction is executed.
Through the above processes, even in the presence of errors on axes, measurement and correction of the errors can reduce geometrical errors in CMM and provide CMM with high precision.
In apparatus such as CMM, a base also serving as a Y-axis motion guide plays a particularly important role in geometrical errors. Volumetric compensation can be utilized to provide CMM with high precision. In this case, if the base also serving as the Y-axis motion guide has a geometrical error, the base deforms when a user work is mounted thereon. As a result, the Y-axis motion guide is given a variation in geometrical precision, which leads to deterioration of CMM precision. Therefore, the base is designed to have a larger thickness in the art to improve the static stiffness of the base to increase the user's maximum loading weight. Accordingly, consideration is required for the thickness of the base on a basis of the maximum loading weight for a user work. This results in a longer delivery time and a larger cost. In particular, the recent increased use in measurement of large mold works desires CMM capable of measuring a 5 ton-10 ton work.
The present invention has been made in consideration of such the point and accordingly has an object to provide a coordinate measuring machine capable of achieving measurements with high precision depending on user works and method of correcting a measuring space without alternation of the thickness of the base.